1. Field of the Disclosure
The disclosure relates to powerful fiber lasers configured to substantially decouple parasitic pump signals from an inner cladding of multiclad output fiber.
2. The Prior Art Discussion
High power lasers are used for a variety of applications such as cutting and welding of metals. Among the most important parameters determining the usefulness of the laser source for these applications are the total optical power and the beam quality of the laser on the surface of the material. A low quality laser beam cannot be focused into a small spot and as a result the power density is insufficient for thermally induced changes in the material. Alternatively a high beam quality laser may be focused into a small spot but the beam diverges rapidly, making it hard to keep the material in focus.
The beam quality of light propagating along a fiber can be markedly improved if only the light supported by the core of the fiber is radiated. Unfortunately, in fiber lasers, the cladding of fiber may also support light at wavelengths either identical to those supported by the core or different therefrom.
The former can be observed, for example, in a combiner in which several fiber lasers are coupled together into a single fiber output fiber. Not all of the desired light is coupled into the core of the output fiber; some of it is supported by cladding and should be removed therefrom.
The light beams supported by a core and cladding of a fiber system, respectively, at different wavelengths are, for example, pump light propagating along the cladding and signal light guided in the core. If not substantially removed from the fiber, the light guided outside by cladding, may and will affect the quality of the signal light.
One of examples illustrating the above relates to a fiber laser configured with multiple claddings. The dual cladding fiber is comprised of a fiber with a small rare earth doped core, a surrounding undoped inner cladding and an outer cladding. Several different rare earth dopants may be used including, among others, Yb, Nd, Er and Tm. The outer cladding may be a lower index glass or polymer. Pump light with wavelength around the absorption band of the rare earth dopant is injected into the inner cladding and, while propagating through the inner cladding, excites the rare earth providing gain in the core area. The major advantage of the multiclad fiber, over the more traditional core pumped variety, is the large pump area and high numerical aperture. A low index outer cladding increases the NA of the inner cladding and permits collection of more pump light in the inner cladding. Single mode cores provide the highest beam quality. The beam quality of such lasers typically surpasses the requirements imposed by a variety of fiber laser applications.
When scaling to higher powers, it is important to optimize fiber for efficient coupling of the pump light, reduction of non-linear effects, high-conversion of pump light and good thermal properties. In terms of fiber features, this can be expressed, among others, as:                High numerical aperture;        Large core size;        Short fiber length; and        Low residual pump power.        
FIG. 1 illustrates a powerful laser system 10 including an array of single mode lasers 12. The SM lasers are coupled together in a multimode combiner 14 having a MM output 16 guiding a pump signal, which can reach the order of kW, at a pump wavelength λp. Having coupled to one another in combiner 14, lasers 12 generate pump light filling up the interior of combiner output 16 so that only one radiation mode is coupled into and supported by a core 19 of multiclad passive fiber 18 at the pump wavelength.
The multiclad fiber 18 couples the pump light into a rare-earth doped active fiber 22 which has a multiclad configuration and can operate either as a laser or an optical amplifier. A delivery passive multiclad fiber 24 is fused with the downstream end of fiber 22. A combination of multiclad fibers 18, 22 and 24 defines a gain block component of fiber system 10. If active fiber 22 is configured as an oscillator, fiber gratings 20 and 23 written, for example, in the cores of respective passive fibers 18 and 24 define a cavity therebetween and configured so that the laser signal is generated at desired wavelength λs different from wavelength λp of the pump signal. The end-pump configuration illustrated in FIG. 1 provides for a signal which is (a) powerful because of multiple pump lasers 12 and (b) high quality since core 19 of multiclad fibers 18, 22 and 24 supports substantially only a fundamental mode.
Multiclad fibers 18, 22 and 24 are not truly single mode fibers. In general, there are two types of modes in these fibers: a fundamental mode associated with the “single mode” core guided at desired signal wavelength λs, and a number of higher-order modes guided by the outer boundary of the multimode waveguide at pump wavelength λp. The output of the dual cladding fiber is complicated. Although the majority of the power is in the single mode core, there is enough power in higher order modes supported by the inner cladding.
The active fiber 22 should be of a substantial length in order to adequately absorb the powerful pump signal. Such a length, however, may be associated with at least two undesirable consequences. First, the greater the length, the greater the power loss due to nonlinear effects. Second, the greater the length, the father away the laser signal wavelength drifts from the desired one. Typically, even if the length of active fiber 22 is optimized, about 10% (about 10 dB) of the pump signal is still guided in the cladding of delivery fiber 24 at pump wavelength λp. As a consequence, at the output of system 10 two signals—one at the desired wavelength and one at the parasitic pump wavelength—are emitted. Needless to say, the parasitic pump signal deteriorates the quality of the main/laser signal and, in fact, is simply unacceptable in many applications. The length reduction however leads to a substantial residual pump power in the cladding that could not be absorbed in the relatively short core.
The problem, discussed above, has been addressed in U.S. Pat. No. 7,349,596 which is fully incorporated herein by reference. The patent discloses a multiclad fiber including a core, inner cladding and outer cladding which are configured concentrically with one another. The outer and inner claddings have respective refractive indexes which substantially match one another. As known to one of ordinary skills in the fiber laser art, light travels between contacting mediums when the latter have respective indices which substantially match one another. The coupling of light is associated with elevated temperatures and continues until the refractive index of the outer cladding lowers below than that one of the inner cladding.
Since the indices of the respective inner and outer claddings as disclosed in the patent match each other, the light guided in the inner cladding may not be fully decoupled therefrom. The effective decoupling of light in accordance with the teaching of the patent is associated with the necessity of maintaining a desirable temperature of heat sink. To maintain the desired level of temperatures, the patent discloses a complicated cooling system rendering the disclosed configuration structurally complicated and cost-ineffective. Still a further concern associated with the matching indices is rather a great overall length of the light stripper sufficient for removing substantially the entire cladding-guided light.
A need, therefore, exists for a light stripper operative to effectively remove light from the cladding of a fiber waveguide.
A further need exists for a high power fiber laser system provided with a light stripper operative to remove pump light from the cladding in a cost-effective and simple manner.